The invention relates to a method for determining the tread depth of a vehicle tire, with the tire being mounted on a vehicle, the tire being rolled over or placed on a measuring station, the tread of the tire being optically sensed transversely to the rolling direction of the tire on at least one measuring line, a ray fan extending from a light source being reflected at the tire surface and a signal of the reflected ray fan being recorded by a sensor, and the signal being evaluated. The invention further relates to an apparatus for determining the tread depth of a vehicle tire, comprising a measuring station which includes a light source and an image-resolving sensor, as well as comprising an evaluation unit for signals of a ray fan reflected at the tire surface.
The method as described enables a measurement of the tire tread depth while a vehicle is running over a stationary sensor station. Thereby, the entire cross profile of the tire is detected, but only a portion of the circumference of the tire.
Currently usual methods for measuring the tread depth while passing over a station are based on that the tread depth is measured exactly when the tire is placed with its foot print horizontally over the sensor. This has the advantage that the tire tread stands statically over the contact surface during a short time interval and can be determined using contacting methods or can be sampled using contactless methods, such as ultrasonics, radar reflectivity or other optical methods, for example lasertriangulation or light section processes.
The light section measuring technique is in particular advantageous for a quick three dimension detection of objects. Thereby, a laser using a specific expansion optics projects a light line onto the surface of the tire. Systems working with light section measuring technique are utilized in all stages of tire production until inspection of the finished product. The invention, however does not mean such an inspection of the tire during its fabrication. Moreover, the invention seeks to perform a tire inspection at a tire mounted on a vehicle, for example in flowing traffic without the traffic flow itself being influenced.
DE 43 16 984 A1 describes a method and a device for automatically determining the tread depth of vehicle tires mounted on the vehicle. In the flow of a measuring station a partly transparent measuring plate is arranged, with a measuring head being placed therebelow. This measuring head comprises a laser and an image-resolving sensor as a triangulation unit. For measuring the tread depth of a vehicle tire, the measuring plate is rolled over by the tire or the tire is placed on the measuring plate. Then, the laser creates a light spot on the tread surface of the tire. The position of the light spot is monitored by the sensor. The output signals of the sensor are passed to an evaluation unit which determines the dimensions of the tire tread. The measuring head is arranged at a carriage and can be moved transverse to the rolling direction of the tire. To avoid a faulty measuring result, the laser is oriented such that the laser beam impinges onto the tire tread outside the foot print of the tire. Within the foot print, the tire is considerably loaded by tire load and the tire pressure, so that the knobs of the tire profile are compressed in radial direction of the tire so that the relevant profiled contour is possibly not recognizable at all and the tread depth is evaluated too small. The measuring result does no longer correspond to the manual measurement which is seen a reference. Since this effect at least depends on the material properties of the rubber, on the tire tread, on the tread depth, on the tire pressure and on the tire load, it cannot be determined or compensated. Outside the foot print, on the other hand, measuring is done practically without any load.
However, point measurements, as performed according to DE 43 16 984 A1, have the disadvantage that there is no relationship to the outer contour of the tire. If the sensor measures within a tread groove, it has no information about the height of the reference surface and, therefore, cannot achieve at precise results. This, in turn, can only be avoided by contact of the tire which however involves the disadvantages previously described.
In this method, moreover, it is measured only along a line of the tire tread transverse to the rolling direction of the tire. Thereby, only chance will decide whether the tread depth at this position of the tire is representative for the tread depth of the entire tire. It is also not possible to select a specific position of the tire tread surface to perform the measurement, since it is completely random which part of the tread surface is currently located over the measuring head. DE 43 16 984 A1 attempts to address this problem by providing several measuring units consecutively arranged in direction of motion.
WO 98/34090 solves a part of the problems addressed above in that the profile surface of the vehicle tire is charged by a linear laser beam. The light reflected back from the tread surface of the tire is detected by an image-resolving sensor, which signals are processed for producing output data in accordance with the tread depth. Thereby the vehicle tire which engages at least one rotatable roller is rotated during the measurement so that measuring can take place at several positions of the tire tread. Furthermore, forces shall become effective by rotating the tire, so that contamination and loose stones are removed from the tread surface. Since this method is to be used in a garage, for example during braking tests, the system cannot necessarily be used in a traffic situation wherein, depending on weather conditions, permanently contamination and also moisture can dominate which interfere a measurement, since for example wet tires produce large inaccuracies caused by reflections.
DE 103 13 191 A1 describes a method of contactless dynamical detection of the profile of a solid body, in particular a vehicle tire. Determination of the profile occurs by means of triangulation wherein correction values determined from the moving velocity of the vehicle tire are considered.
DE 18 09 459 A1 discloses a method and an arrangement for measuring the tread depth of vehicle tires the loaded tread is measured. According to EP 0 869 330 A2, the tire is rotated through rollers during measuring the tire profile by means of triangulation. The latter is also disclosed in WO 98/34090, wherein the sensor receiving the reflected signal is oriented orthogonally to the tire surface. Also EP 0 816 799 A2 describes a method wherein the tire is rotated by rollers. Determination of tread depth occurs also by means of triangulation.